1. Field of the Invention
The present invention relates to an image reading apparatus capable of simultaneously reading both sides of a double-sided document using a first reading unit and a second reading unit different from the first reading unit.
2. Description of the Related Art
As a conventional reading function of electronically reading image information of a document, a copying machine, a facsimile apparatus, a multi-functional peripheral having a facsimile function, and a scanner serving as an image reading apparatus for computer input are popular. These image reading apparatuses are classified into two types: an image reading apparatus of one type reads a document while the document while it remains in one position, and an image reading apparatus of the other type reads a document while moving it. Examples of the latter type are a reading apparatus which automatically conveys a plurality of document sheets to continuously read them, and a reading apparatus which automatically reverses a double-sided document within the apparatus to read both sides of the double-sided document without a user's manipulation.
The double-sided document means a document bearing image information such as text and/or image data on both sides of the document, i.e., the front-side and back-side of the double-sided document.
In addition, as devices achieve more compact sizes and higher performance, reading apparatuses which simultaneously read the both sides of a double-sided document without reversing have recently become available. Such a reading apparatus is a simultaneous double-sided image reading apparatus having two reading devices; one for the front-side and one for the back-side.
When simultaneously reading pieces of image information on both sides of a double-sided document, the simultaneous double-sided image reading apparatus has the following advantages: it does not require efforts of the user, it can shorten the reading time in comparison with a reading apparatus which reverses a document, and it can reduce the occurrence rate of a jam because it uses only one document conveyance path.
At the same time, however, the simultaneous double-sided image reading apparatus has the following disadvantages. Since the front-side and back-side are read by different reading devices, the simultaneous double-sided image reading apparatus suffers the individual differences of the reading devices, a difference in the degree of deterioration of durability upon use for a predetermined period, and the like. The MTF (Modulation Transfer Function) also changes due to the difference in the degree of float of a document from the conveyance path at the reading position. As a result, characteristics such as the tints and geometrical characteristics of the read images of two sides may differ from each other. These characteristic differences become more serious in a simultaneous double-sided image reading apparatus using devices of different systems, e.g., the reduction optical system (CCD system) and equal-magnification optical system (CIS system) for the front-side and back-side.
If the reading characteristic of one image reading apparatus differs between the front-side and the back-side, the user will hesitate to use the apparatus. To prevent this, an image reading apparatus capable of double-sided reading needs to make at least front-side and back-side reading characteristics coincide with each other.
For example, Japanese Patent Laid-Open No. 2007-082033 proposes an arrangement which solves the above-described characteristic difference as follows. A CIS reading device for reading a front-side is added in a double-sided reading device including a CCD reading device for reading the front-side of a document, and a CIS reading device for reading the back-side. At the same time, a back-side path mechanism is arranged. The CCD reading devices read both the front-side and back-side, eliminating the difference between front-side and back-side reading characteristics.
However, the arrangement described in Japanese Patent Laid-Open No. 2007-082033 for solving the problem cannot completely prevent the difference between front-side and back-side reading characteristics that is generated due to the individual differences of the reading devices, a difference in the degree of deterioration of durability, and the like. Further, the arrangement described in Japanese Patent Laid-Open No. 2007-082033 goes against the current trend toward more compact, lighter-weight devices. The solutions of adding a reading device and arranging a back-side path are not optimum in the current development situation requiring cost reduction and space saving.
One method for solving the above-described problem is calibration. Calibration makes a plurality of device characteristics coincide with each other, and various charts and correction methods have been devised for calibration. Japanese Patent Laid-Open No. 2005-304092 proposes calibration which makes the reading characteristics of the front-side and back-side coincide with each other by reading a double-sided linear-density chart document and correcting at least the reading characteristic of one surface. However, according to the method in Japanese Patent Laid-Open No. 2005-304092, one surface is used as a reference to correct the reading characteristic of the other surface. For the corrected surface, the tint of an image obtained as a reading result greatly changes before and after calibration.
This problem will be explained in more detail with reference to FIGS. 2A and 2B.
In FIGS. 2A and 2B, the abscissa axis represents the time, and the ordinate axis represents the degree of color shift from a document. A solid line represents a change of the reading characteristic of the first reading device 110 (referred to hereinafter as a “front-side reading device”), and a broken line represents that of the reading characteristic of the second reading device 111 (referred to hereinafter as a “back-side reading device”). The front-side reading device is used in both single-sided reading and double-sided reading, and considered to deteriorate more than the back-side reading device.
FIG. 2A shows a case where correction is done using the back-side reading device as a reference in calibration. The reading characteristic of the front-side reading device is greatly improved, and the reading characteristics of the front-side and back-side reading devices can be made to coincide with each other. However, the tint becomes greatly different between single-sided copy (1) immediately before calibration and single-sided copy (2) immediately after calibration.
FIG. 2B shows a case where calibration is done using the front-side reading device as a reference. The reading characteristics of the front-side and back-side reading devices coincide with each other, and the difference in tint between single-sided copy (1) and single-sided copy (2) is also suppressed. However, as described above, the front-side reading device is used in both single-sided reading and double-sided reading, and is used more frequently than the back-side reading device. The durability is considered to deteriorate more. When correction is done using the front-side reading device as a reference, the precision of color processing is adjusted to a lower one. That is, the color differs more from a document.
In FIG. 2A, even when the same document as that before several hours or several days is single-sided-copied again, an output result different in tint is obtained. A user who usually uses single-sided copying will find the output results to be unnatural. In FIG. 2B, color matching between the front-side and back-side reading devices is implemented. However, the reading characteristics of the front-side and back-side reading devices are uniformly impaired, failing to achieve the purpose of calibration.